The present invention relates to a rotating electric machine with an external rotor, and more particularly to an electric motor with an external rotor cage.
Rotating electric machines with external rotors, and in particular asynchronous electric motors with external cage rotors, are known. The purposes of this constructive solution are well known and dictated by the requirements of the particular applications (such as, in particular, axial and centrifugal fans mounted directly on the rotor frame, devices which require a high inertia of the rotating masses and an enhancement of the self-ventilating effect, etc.).
A typical known form of construction of such a motor with an external rotor, given by way of comparison in FIG. 1, which will be discussed in greater detail hereinafter, comprises a supporting axial element constituted by a flanged bush which is protrudingly mounted on a stationary supporting shield, and externally supports the active stator parts and internally a coaxial shaft, mounted rotatably on bearings also internal to the bush, and with a shaft end protruding from the bush. On the shaft end which protrudes from the bush there is keyed the opposite shield, protrudingly and monolithically supporting the rotor frame and the related active parts contained therein ("rotor cage" and lamellar core).
Such a construction of the prior art has several disadvantages among which are the following:
difficult centering and critical plays of the rotating masses, due to the protruding mounting of the external rotor part on the shaft end protruding from the bush;
high stress and wear of the bearings, consequent to the protruding mounting of the rotating structure and to the concentricity errors deriving therefrom, with the aggravation that the need to contain the bearings inside the bush limits their size, and forces their operation in a hot environment and in an axially close mutual position;
noise, due to the above reasons;
need for forced mounting of the stator pack on the bush, with consequent deformation of the bearing seats;
oversizing of the diameter of the bush, which must contain the shaft at the expense of the active stator material;
plurality of parts, and of mechanical machinings of the related coupling surfaces.